Multithreading is the ability of a program or an operating system to execute more than one sequence of instructions at a time. Each user request for a program or system service (and here a user can also be another program) is kept track of as a thread with a separate identity. As programs work on behalf of the initial request for that thread and are interrupted by other requests, the status of work on behalf of that thread is kept track of until the work is completed.
Types of computer processing include single instruction stream, single data stream, which is the conventional serial von Neumann computer that includes a single stream of instructions. A second processing type is the single instruction stream, multiple data streams process (SIMD). This processing scheme may include multiple arithmetic-logic processors and a single control processor. Each of the arithmetic-logic processors performs operations on the data in lock step and are synchronized by the control processor. A third type is multiple instruction streams, single data stream (MISD) processing which involves processing the same data stream flows through a linear array of processors executing different instruction streams. A fourth processing type is multiple instruction streams, multiple data streams (MIMD) processing which uses multiple processors, each executing its own instruction stream to process a data stream fed to each of the processors. MIMD processors may have several instruction processing units, multiple instruction sequencers and therefore several data streams.
The programming model adopted by today's multithreaded microprocessors is the same as the traditional shared memory multiprocessor: multiple threads are programmed as though they run on independent CPUs. Communication between threads is performed through main memory, and thread creation/destruction/scheduling is performed by the operating system. Multithreading has not been provided in an architecturally-visible manner in which programmers can directly access threads.